In the manufacture of a shaped or monolithic basic refractory for lining or repairing high temperature furnaces and kilns, the bricks, ramming or gunning mixes are customarily made from deadburned magnesia or dolomitic grains. Ideally, the assorted particles (grains) from which a refractory shape is manufactured should be composed of fine crystals of the metallic oxides. Mineralizers or other modifiers such as chrome ore may be present as components of the grains.
Fine, tight crystal formation within each grain of a refractory serves to prolong refractory life by reducing slag and metal intrusion and erosion. In addition, the assorted grains from which the refractory bricks or monolithic linings are made should be composed of an assortment of grain sizes and shapes which will pack tightly together and minimize slag and metal intrusion at the grain boundaries.
A grain assortment composed entirely of sized spheres has an ideal shape factor and will pack to leave a minimum of interstitial voids. It is, therefore, the ideal grain shape for forming refractory brick and shapes and monolithic linings and repairs. A number of processes for the production of spherical grains have been suggested and utilized. By way of example, spherical grain production has been accomplished by the introduction of magnesium hydroxide filter cake produced from sea water directly into a rotary kiln where the material rolls into essentially spherical pellets as it is being calcined. These spheres can be deadburned in the same kiln to produce a useful, but undesirably porous, refractory grain. To reduce porosity, the spherical nodules as calcined can be crushed and reformed into briquettes and then deadburned in a rotary or vertical kiln to form a more acceptable refractory grain. The deadburned briquettes must then be crushed to meet sizing requirements resulting in an angular grain assortment.
Another method for producing spherical pellets is to use a pan or disc pelletizer where calcined oxide powders are rolled into spheres much in the manner of rolling a miniature snowball. In this process a liquid such as water is utilized as a starting nucleus and to further wet the surface of the forming pellet to continue the accretion process to a desired pellet size.
Each of the above-mentioned processes makes spherical magnesia grains which, after deadburning in commercial kilns, produces grains having densities which approach, but do not reach, 3.4 grams per cubic centimeter. Each type of spherical pellet also has the common deficiency of an annular or a "cabbage leaf" cross-section which upon deadburning leaves voids. Further, the pellets prepared by the above-described process are prone to disintegration before they are deadburned and make alternative processing steps or other end use applications inefficient or impractical. Consequently, attempts to produce spherical refractory grain by pan pelletization or rotary kiln tumbling have enjoyed little commercial success in meeting the more rigorous refractory lining requirements.
Most refractory grain for use in the lining of oxygen converters for steel making is now made from natural or synthetic magnesium oxide which may originate as magnesite, dolomite, brucite, or magnesium chloride in sea water, bitterns or brines.
In the case of the natural materials, the carbonates or hydroxides may be deadburned in a rotary kiln or calcined first in a multiple hearth furnace or rotary kiln and the calcine is then fed to a briquetting press for shaping into a granule which can be further fired in a rotary or vertical kiln to produce dense refractory grain. In either instance, the size assortments necessary for brick forming are produced by crushing and screening the fired deadburned granules or briquettes.
Where sea water or brines are the starting material, magnesium hydroxide is formed from magnesium chloride by the wellknown chemical replacement technique, and the insoluble magnesium hydroxide slurry is collected as filter cake. The filter cake is calcined in rotary kilns or multiple hearth furnaces and the resulting oxide dust is briquetted and the briquettes are deadburned at high temperatures in rotary or vertical furnaces to produce grain after crushing and screening in the same manner as described previously for the natural materials.
The density of the grain produced from many of the briquette precursors may be good, but is always limited to angular shaped particle assortments which when made into a refractory will pack leaving more interstitial voids than would an assortment of spherical grains.
U.S. Pat. 3,666,851 describes a process for preparing high density magnesia refractories utilizing a hot pressing technique. A mixture of magnesium oxide and magnesium hydroxide is heated and hot pressed while the magnesium hydroxide decomposes to the oxide. Sintering of the hot pressed specimens produces final densities supposedly as high as 98% of the theoretical value.
Another method of preparing dense deadburned magnesia pellets is described in U.S. Pat. 2,348,847. This patent describes a procedure wherein a mixture of reactive magnesia and magnesium hydroxide is formed into nodules utilizing an extrusion process. The nodules are advanced to a device where they are tumbled to round the edges and where an exothermic reaction occurs resulting in the formation of round, hard, strong pellets. The pellets are subsequently deadburned to magnesia having a density in excess of 3.00.